In wireless a communication system, due to the channel impairments, wireless links require Automatic Repeat ReQuest (ARQ) protocols to ensure that data is transmitted reliably and between a transmitter and a receiver in network devices. In ARQ protocols, the transmitter transmits a frame, or encoded parts of the frame, as many times as required for the receiver to correctly decode it. A frame refers to data bits, which are delimited with a clear start and an end in the physical layer, or higher layers. In higher layers, a frame is often referred to as a packet, while in lower layers the frame is sometimes referred to as a transport block (TB). The three terms: a frame, a packet, and a TB are used interchangeably in this paper.
ARQ protocols can be classified into pure ARQ protocols and hybrid ARQ protocols. In pure ARQ protocols, the transmitter retransmits the original transmitted frame in each retransmission, while the receiver decodes each received frame independently of other received frames. In hybrid ARQ (HARQ) protocols, the receiver decodes groups of received frames jointly using soft combining. In one version of HARQ protocols, transmitter transmits the same frame in each retransmission, in which case the receiver uses chase-combining (CC) to decode the frame. In another version of HARQ protocols, the transmitter sends the frames using incremental-redundancy (IR), where the original frame was encoded into a much longer sequence of coded bits than the original frame. Each transmitted frame is transmitted with different set of coded bits. The receiver uses all received coded bits to recover the original data. A combination of both CC and IR methods is possible.
In the current 3rd Generation Partnership Project Long-Term (LTE) Evolution standard, the MAC HARQ (3GPP, 2012) does not provide reliable delivery of upper layer packets. The reliable and in-sequence delivery of packet is provided in the radio link control (RLC) layer (3GPP, 2010). The two-layer approach introduces significant signaling overhead and may also increase the latency of TB transmissions.
In the LTE standard, HARQ uses a single-bit ACK to indicate that a TB was successfully decoded at the receiver. The same bit is also used as NACK to indicate that a TB was not successfully decoded. Upon detecting an ACK, the transmitter moves on to transmitting a new TB. On the other hand, upon receiving a NACK, the transmitter transmits the next available Incremental Redundancy (IR) version of the original TB. The transmitter keeps transmitting IR versions of the TB until it receives an ACK from the receiver. In a protocol with a reliable delivery, the transmitter should only move on to transmitting the next TB after the receiver has successfully decoded the current TB. The HARQ protocol in the current LTE standard, however, cannot guarantee a reliable delivery of TBs.
In the case of LTE HARQ, the transmitter may move on to transmitting the next TB even if the receiver has not successfully decoded the previous TB. A TB that was not successfully decoded is referred as a “lost” TB. LTE HARQ protocol therefore requires a secondary level of ARQ in the upper layers to ensure that the lost data bits are actually delivered. The upper layer automatic repeat request (ARQ) is provided in the radio link control (RLC) layer. This solution suffers from unnecessary overhead included in the RLC layer, as well as unnecessary delays caused by extra processing. While LTE's protocol design uses a spectrally efficient control signaling in the physical layer, the two-layer approach substantially increases the amount of control signaling in the upper layers to deliver upper layer packets reliably.
One way that TBs are lost is when channel impairments change the bit indicating a failed transmission (NACK) into a bit indicating a successful transmission (ACK). The transmitter interprets the received ACK as an indication of successful decoding at the receiver and proceeds to transmit the next frame. The receiver is required to move on to the next TB and data bits are lost for upper layers. In the current HARQ protocol the transmitter has no way of detecting if this has happened and is thus unable to guarantee that all upper layer packets are delivered reliably. We note that due to channel impairments a transmitted ACK may be interpreted as a NACK at the transmitter, which will cause unnecessary frame retransmissions with a decrease in spectral efficiency, but it will not cause loss of data bits.
Another way that TBs are lost is if channel impairments cause the transmitter of the frame to interpret a discontinuous transmission (DTX) by the receiver as an ACK. For example, due to incorrect decoding of the resource allocation information in the physical downlink control channel (PDCCH), the receiver may not attempt to decode a TB and does not transmit anything in response to a downlink frame. The transmitter should interpret the lack of response by the receiver as a discontinuous transmission (DTX) and therefore detect the lost PDCCH information. The TB can then be retransmitted. However, due to the noise at the transmitter DTX may in fact be interpreted as an ACK, causing the transmitter to move on to the next TB. As with the ACK to NACK error, this problem is solved with the upper layer ARQ in the RLC layer. Similarly, the transmitter of the frame may also detect DTX as a NACK, in which case it would transmit the next IR version of the frame. While this event may not result in a lost frame, it increases the latency of decoding the TB.